Ecological correlates of trophic polyphenism in spadefoot tadpoles inhabiting playas

Polyphenism in larval amphibians has been related to several factors, including wetland hydroperiod, food availability, competition, and predation. Spadefoot toads (genus Spea Cope, 1866) often exhibit a trophic polyphenism by developing distinct carnivore and omnivore larval morphotypes. Using a multimodel selection approach, we investigated the influence of land use (cropland vs. grassland type) and differences in annual precipitation on morphotype expression in Plains spadefoot (Spea bombifrons (Cope, 1863)) and New Mexico spadefoot (Spea multiplicata (Cope, 1863)) toads in playas. We also examined the relative importance of tadpole density, tadpole age, water-loss stress, wetland size, density of larval mole salamanders (genus Ambystoma Tschudi, 1838; a predator on Spea tadpoles), and food resources on morph occurrence. The carnivore morphotype developed almost exclusively in S. bombifrons and rarely in S. multiplicata regardless of land use. Habitat availability measured by water-loss rate, as well as predation risk and tadpole age, were the most important factors influencing carnivore proportions. Ambystoma density was positively associated, whereas water-depth loss and tadpole age were negatively associated with the proportion of carnivores. The greatest proportion of carnivores was observed in grassland playas, which had the highest density of Ambystoma predators, longest hydroperiods, and experienced water-depth gain. Fairy shrimp density was not correlated with the proportion of carnivores. Upland land use through cultivation-associated erosion is altering wetland trophic structure, which further influences morphotype expression in Spea tadpoles and playa amphibian community structure.